Abstract
Histones specifically phosphorylated at either tyrosine (phospho-Tyr-histones) or serine and threonine (phospho-Ser-histones) were prepared and tested as substrates for so-called alkaline and protein phosphatases. Three different preparations of alkaline phosphatase (calf intestine, bovine liver, and Escherichia colg dephosphorylated phospho-Tyr-histones at 5-10 times the ratet hey dephosphorylated phospho-ser-histones. It was concluded that the p-nitrophenyl phosphatase and phospho-Tyr phosphatase activities reside in the same protein on the basis of high performance liquid chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, inhibition of both activities by Pi and EDTA but not fluoride, and inhibition of phospho-Tyr phosphatase activity by p-nitrophenyl phosphate. Phosphoprotein phosphatase from rabbit muscle, which had little if any p-nitrophenyl phosphatase activity, dephosphorylated phospho-Ser-histones at 16 times the rate it dephosphorylated phospho-Tyrhistones. Thus, the ratio of the pseudo-first order rate constants for dephosphorylation of phospho-Tyr-histones/ phospho-Ser-histones was approximately 100 times higher for alkaline phosphatases compared to the rabbit muscle protein phosphatase. Membrane proteins from A-431 cells (phosphorylated at tyrosine) were more effective substrates than phospho-Tyr-histones as substrates for alkaline phosphatase; this was not the case for protein phosphatase. Phospho-Tyr phosphatase activity of the alkaline phosphatases was optimal in the pH range of 7-8. The so-called alkaline phosphatases, then, may be a group of membrane-bound glycoproteins that represent a class of phosphoprotein phosphatases that show selectivity for proteins phosphorylated at tyrosine residues.
Published Version
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